The overall objective of our project is to elucidate further the influence of thyroid hormone on skeletal muscle growth in chickens. We are interested in differences in the actions of thyroid hormones during the second half of embryogenesis and during early post-hatching growth. Our studies will focus on processes which underlie the nearly two-fold increase in muscle weights between 17 and 20 days of embryogenesis, which are prevented by goitrogen treatment. The goals of our proposed research for the coming year are the following: 1. Study of thyroid hormone action. Experiments will be done to ascertain the activities of RNA polymerases I, II, and III for muscle nuclei isolated from embryos with altered thyroid status. If there is a significant increase in polymerase I activity within 24 hr after the administration of thyroid hormone, we will investigate the incorporation of 3H-CTP into 40S ribosomal subunit, which can be isolated from other RNP components. The experiments in embryos will be followed by comparable experiments in early post-hatching chickens to ascertain whether there are differences in the responsiveness to thyroid hormone. 2. T3 binding to saturable receptor sites. Studies will be done to gain information regarding T3 binding to isolated muscle nuclei. 3. Studies in dystrophic chickens. An experiment will be done to determine whether goitrogen treatment, which reduces nuclear proliferation in muscles of normal chickens, will block the abnormal nuclear proliferation and accompanying increase in 6PGD activity during the onset of muscular dystrophy. BIBLIOGRAPHIC REFERENCES: King, D.B. and King, C. R. 1976. Thyroidal influence on gastrocnemius and sartorius muscle growth in young White Leghorn cockerels. Gen. Comp. Endocrinol. 29:473-479. King, D.B., King, C.R., and Eshleman, J.R. 1977. Serum triiodothyronine levels in the embryonic and post-hatching chicken with particular reference to feeding-induced changes. Gen. Comp. Endocrinol. (In press).